Use ScopedTaskEnvironment instead of MessageLoop in third_party unit tests.

ScopedTaskEnvironment allows usage of ThreadTaskRunnerHandle and
base/task_scheduler/post_task.h within its scope. It should be
instantiated in every unit test that uses either of these APIs
(i.e. no test should instantiate a MessageLoop directly).

Motivation for ScopedTaskEnvironment can be found in:
https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit

BUG=708584
R=gab@chromium.org
TBR=satorux@chromium.org

Review-Url: https://codereview.chromium.org/2797313002
Cr-Original-Commit-Position: refs/heads/master@{#462497}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2d4bf869bc8ba9245190e1a5e6e72328afd90ef0
1 file changed